1. Field of the Invention
This invention relates to a solid-state image sensor driving device and more particularly to a device for driving an electronic shutter.
2. Description of the Related Art
Heretofore, video cameras or the like have been arranged to perform an exposure compensating function by various methods, including a method of directly controlling the quantity of light incident on an image sensor by means of diaphragm blades or the like; and another method of performing the function of the so-called electronic shutter whereby the electric charge accumulating time of the image sensor is controlled.
However, these conventional methods have presented the following problems: the method of using the diaphragm blades necessitates the diaphragm blades and related mechanical members. In the case of the method of performing the electronic shutter function, it tends to give an unnatural picture movement. Of these problems, the details of the problem presented by the method of performing the electronic shutter function are as follows:
FIG. 1 of the accompanying drawings conceptually shows an inter-line transfer type CCD image sensor. Referring to FIG. 1, the illustration includes a sensor part 1 which is arranged to perform photoelectric conversion; a vertical transfer register 2; a horizontal transfer register 4; and an output amplifier 5. FIG. 2 is a sectional view taken on a line A--A' of FIG. 1. FIG. 2 also shows potentials obtained at the image sensor.
Referring to FIG. 2, the illustration includes a channel stop 6 (hereinafter abbreviated to CS) which is arranged to separate picture elements; a read-out gate 7 (abbreviated to ROG) which is provided for moving the electric charge accumulated at the sensor part 1 to the vertical transfer register 2; a substrate 8; and an oxide film 9.
The following describes the electronic shutter function of the video camera with reference to FIGS. 2 and 3: FIG. 3 shows one-field amount of a standard TV signal. Referring to FIG. 3, a pulse .phi.ROG is arranged to be applied to the read-out gate (ROG) 7. When this pulse is at a logical level H (high), the potential of the read-out gate 7 decreases to allow the electric charge of the sensor part 1 to be moved to the vertical transfer register 2. A removing pulse .phi.SUB is arranged to be applied to the substrate 8. When that pulse is at the level H, the electric charge accumulated at the sensor part 1 is swept out to the outside through a .phi.SUB terminal.
In FIG. 3, the conventional device described above is shown in a state of having the pulse .phi.ROG in a vertical retrace line period and the pulse .phi.SUB in a horizontal retrace line period. A next period begins after the electric charge of the sensor part 1 is read out at a point of time t0. However, since the pulse .phi.SUB comes to be at the level H during a horizontal retrace line period up to a time point t1, the electric charge obtained during a period between the time points t0 and t1 is not remaining at the sensor part 1. During an ensuing period between time points t1 and t2, the pulse .phi.SUB is at a level L (low). During this period, therefore, the electric charge is accumulated at the sensor part 1 until the time point t2. At the time point t2, the level H of the pulse .phi.ROG causes the electric charge to be shifted to the vertical transfer register 2. In this instance, the exposure time of the camera is "t2-t1".
The conventional device thus adequately performs the function as an electronic shutter. However, in cases where the conventional electronic shutter function is to be used for continuously changing the exposure time for exposure correction, a difference in dynamic resolution between one exposure time and another appears on a picture. This tends to greatly impair the picture quality.